The rapid charging of batteries, and in particular lead-acid batteries, has been pursued for decades. Procedures for rapid charging of batteries have been reported over the past 20 years or so, mainly for Ni--Cd batteries and, to a lesser extent, for lead-acid batteries. Interest in the latter has increased lately because of intensified interest in electric street vehicles.
All properly designed batteries contain more active material in their plates than their ratings would indicate. In spite of this, most secondary (rechargeable) batteries, especially lead-acid batteries, are routinely used to only about 80 percent of their ratings. Even though only about 80 percent of rated capacity is extracted, the cycle life and lifetime energy throughput is significantly reduced from that of shallower discharges. The cycle life and lifetime energy throughput at 100 percent depth-of-discharge is typically very low.
Conventional charging techniques coupled with rigorous standard discharge conditions often yield a significant amount of cycle-to-cycle capacity variation. Furthermore, grain structure of the active plate material becomes worse and worse with each charging cycle.
A significant amount of research has recently focused on high-rate charging, primarily for rapid recharge (usually partial recharge) for extended range or emergency conditions in street electric vehicles. Until recently, battery charging designers followed the "ampere-hour rule" which holds that the rate of recharge current at any point in the charging cycle should equal the number of ampere hours to be recharged. In spite of this "rule," remarkable side benefits have emerged from studies of high-rate charging--that is, charging rates greatly in excess of that prescribed by the "ampere-hour rule." It appears from the results of these studies that high-rate charging permits greater utilization of active plate material which allows greater depths-of-discharge without detrimental effects and, in fact, is often accompanied by significantly greater cycle life and lifetime energy throughput.
Preliminary physical analyses of high-rate charging effects show: (1) improved maintenance of optimum crystal size within the plate structure, (2) better penetration or use of depth into the third dimension of active plate material, (3) increased electrolyte stirring and convection within local regions and throughout the plate and electrolyte reservoir channels, and (4) enhanced nucleation for crystal formation in deficient plate regions.
Conventional high-rate charging has an objective charging a battery in a small fraction of the time required for conventional charging, through the application of a high rate of current in parallel to all battery cells or cell blocks. Accordingly, a primary drawback of conventional high-rate charging is the extremely high power inputs required. For example, a charger might require 5 kilowatts (220 VAC@23 amperes) during the early part of the charging cycle for conventional charging, but could easily require 50 to 100 kilowatts (440 VAC@113 to 228 amperes) for high-rate charging. Another problem with conventional high-rate charging techniques, which affects battery life, is the inevitable polarization and its concomitant voltage gradients and overvoltages, which usually requires periodic equalization between cells or cell banks. Ideally, some form of equalization should occur during each recharge, but this is usually impractical. Further problems with conventional high-rate charging are a high rate of temperature increase and the possibility of dangerous pressure increase.
Accordingly, it is a principal object of the present invention to provide means and method for high-rate charging that do not necessarily require exceeding the maximum power requirements of conventional (i.e., not high-rate) charging methods.
It is a further object of the invention to provide such means and method that can permit complete charging in less time than conventional charging techniques.
It is an additional object of the invention to provide such means and method that permit utilization of a greater percentage of rated battery capacity, ideally 100 percent of a well designed battery.
It is another object of the invention to provide such means and method that increase cycle life and lifetime energy throughput, to perhaps a doubling or tripling of cycle life and lifetime energy throughput.
A further object of the invention is to provide such means and method which performs equalization (cell-by-cell or block-by-block) during each recharge.
An additional object of the invention is to provide such means and method that improve overall charge coulombic efficiency from, say, 90 percent for conventional charging to greater than 95 percent.
Another object of the invention is to provide such means and method that optimize and synchronize the battery state-of-charge computer employed.
Yet a further object of the invention is to provide such means and method that provide alerts concerning abnormalities, especially equalization imbalance.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.